package LeXin;

import java.text.DecimalFormat;
import java.util.Arrays;
import java.util.Scanner;

public class Main1 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String[] split = scanner.nextLine().trim().split(" ");
        if (split == null || split.length <= 0) {
            System.out.println("0.00");
            return;
        }
        double[] arr = new double[ split.length ];
        for (int i = 0; i < split.length; i++) {
            arr[ i ] = Double.parseDouble(split[ i ]);
        }
        Arrays.sort(arr);
        int jia = arr.length;
        for (double i : arr) {
            if (i <= 0)
                jia--;
            break;
        }
        double[] array = new double[ jia ];
        int index = 0;
        for (int i = arr.length - jia; i < arr.length; i++) {
            array[ index++ ] = arr[ i ];
        }
        if (array == null || array.length <= 0) {
            System.out.println("0.00");
            return;
        }
        double sum = 0;
        for (int i = array.length - 1; i >= 0; i = i - 2) {
            sum += array[ i ];
        }
        DecimalFormat df = new DecimalFormat("######0.00");
        System.out.println(df.format(sum));
    }

}
